U.S. scientists have determined that the Gulf of Mexico’s annual “dead zone” — an area with low oxygen that can kill fish and marine life — is the fourth smallest since they started mapping the area in 1985.

Scientists supported by the National Oceanic and Atmospheric Administration (NOAA) said in a report Tuesday that the area is only about 40 percent the average size predicted earlier this year based on nitrogen and other nutrients flowing down the Mississippi river.

“Although the area is small this year, we should not think that the low-oxygen problem in the Gulf of Mexico is solved. We are not close to the goal size for this hypoxic area,” said lead scientist Nancy Rabalais of Louisiana State University and the Louisiana Universities Marine Consortium.

This year’s dead zone off Louisiana is about 7,040 square kilometers, rather than the 15,000 square kilometers predicted by the NOAA.

Every year the oxygen depletion begins as snowmelt and spring rains bring fresh water to the gulf. Fresh water is lighter than salt water causing two layers to develop. Nitrogen and other nutrients in the fresh water feed a growth spurt of algae and microorganisms at the top. The microorganisms die and fall to the bottom, where their decay consumes oxygen from the bottom up, creating the dead zone.

“The data collected from this annual, long-term research program is critical to our understanding of a wide range of Gulf issues including hypoxia and beyond,” said Steven Thur, director of NOAA’s National Centers for Coastal Ocean Science. “Not only is measuring the size of the Gulf of Mexico dead zone vital to informing the best strategy to reduce its size, but also to reduce its impacts.” …